Same stuff that gives mexican vanilla it's unique flavor.
"its" darnit, not "it's"!
Luckily, I do not touch vanilla, mexican or otherwise. Even among ice cream flavors, it is among my least favored, and has been for years. Something else will get me, but not mexican vanilla.
I always wondered why Martha Stewart had a segment making a dessert, and suggested NOT to use the Mexican Vanilla, but she never said why.